Field
This invention relates to electrical storage battery charging methods and apparatus. Specifically, this invention provides for a method and apparatus to charge liquid electrolyte electric storage batteries by means of pulses of electrical current to increase useable battery capacity and prolong battery useful life.
State of the Art
The amount of energy available from a liquid electrolyte electrical storage battery, the amount of energy returnable after discharge, and the useful life of the battery are all affected by a variety of factors, one of which is the non-homogeneity of the electrolyte, i.e., electrolyte stratification. Specifically, in the absence of agitation or mixing techniques or apparatus, the electrolyte (acid) concentration decreases at the top of a battery cell and increases at the bottom. The electrolyte between adjacent battery cell plates available for discharge and recharge is thus limited, i.e., less than maximum. Cell plate deterioration (e.g., "whisker" or "branch" growths near the bottom of the cell plates; increased erosion at points of low electrolyte concentration) is enhanced by stratification, thereby contributing to a shorter useful battery life than ideally or practically possible.
A variety of agitation and mixing techniques are available to minimize stratification. Included among these are electrolyte mixing pumps for large batteries, platform vibration (e.g., automobile or truck), and battery overcharging procedures. Battery overcharging consists of charging the battery beyond its maximum capacity to effect the electrolysis of the water of the electrolyte solution. Relatively small charging currents are conventionally applied during battery overcharging as compared to typical charging currents and designed maximum charging currents. These small currents, sometimes referred to as the "finishing rate" or "trickle charge" current, may be specified by the battery manufacturer as well as the maximum charging current (rate).
Some of the factors which contribute to the selection of overcharging currents are: the amount of hydrogen and oxygen to be produced during overcharging, battery structure (e.g., cell plate size and material; distance between adjacent cell plates), and electrolyte characteristics, (e.g., concentration). Within the context of imparting the maximum amount of charge, selection is typically premised on a compromise whereby as much gas-induced agitation and mixing as possible is effected without causing excessive erosion in the less-concentrated electrolyte plate area. Overall plate erosion by the hydrogen and oxygen gas as it passes to the surface of the electrolyte must also be minimized. In addition the inherent explosive hazard associated with large amounts of hydrogen and oxygen may of itself be delimiting. In many cases, the net result may be inefficacious because the selected small charging current will not induce uniform mixing. A low battery current tends to flow in the top of the battery where the acid is less concentrated and the electromotive force is lower. Few gas bubbles are then generated in the lower part of the battery. Thus, the lower, more concentrated electrolyte will not be thoroughly mixed with the less concentrated. Therefore stratification is not eliminated, but only reduced.
In many circumstances, vibration, pumping and/or gassing is not practical because of the battery location or use, the cost of apparatus and the explosive dangers associated with hydrogen and oxygen. As a result, reduced battery capacity and reduced battery useful life must be accepted for lack of a practicable alternative.